Gallium-wetted movable electrode switch



April 1968 E. LANGBERG ETAL 3,377,576

GALLIUM-WETTED MOVABLE ELECTRODE SWITCH 2 Sheets-Sheet 1 Filed May 5,1965 A ril 9, 1968 E. LANGBERG ETAL GALLIUM'WETTED MOVABLE ELECTRODESWITCH 2 Sheets-Sheet 2 Filed May 3, 1965 United States Patent Ofiice3,377,576 GALLIUM-WETTED MOVABLE ELECTRODE SWITCH Edwin Langberg,Lexington, and Louis W. Roberts, Boston, Mass, assignors to Metcom,Inc., Salem, Mass, a corporation of Delaware Filed May 3, 1965, Ser. No.452,811 6 Claims. (Cl. 335-496) This invention pertains to electricalswitching devices; more particularly, it pertains to large powercapacity switches which embody arc suppressing features.

Switches in high voltage large current capacity service are subject toarcing between the open switch electrodes. Such arcing phenomenon itselfcauses undesired RF signals, increases switching time, disturbs linebalance heightening undesired line transients, and is destructive of theswitching equipment per se.

The hold-off voltage of a switch is defined as the voltage which theswitch is capable of withstanding with out breakdown or arcing betweenelectrodes when the switch is in an open position. The breakdownphenomenon is caused primarily by ionization of gas molecules in theelectric field between the electrodes. A second breakdown phenomenon,which may be described as electron field emission from the cathodeswitch eledtrode, occurs at higher voltages and also acts to limitswitch hold-off voltage.

At the instant of the contact electrodes separating or closing, when theseparation distances are 10 cm. or less, a very large valued electricfield is created. The cathode, whether it be a solid or liquidelectrode, emits some ionized metal vapor along with substantialthermionic and field emission electron flow sutficient to initiatearcing. If the vapor pressure of the electrode metal is high enough, theare initiated by the breaking of the contacts will continue even afterthe electrodes are separated to their full distances.

Switch arcing pits solid metal electrodes by melting small regions ofthe electrode surfaces and vaporizing or splattering the moltenelect-rode material. The pitted contact electrodes exhibit duringswitching action increased field electron emission due to regions ofintense concentration in the electrical field above the roughened pittedelectrode surfaces. The pitted contact electrodes have higher junctionresistance because of the reduced area of contact. Excessive electrodepitting renders switches useless due to heightened electron emission,overheating and increasingly inefiicient switching action.

Liquid metal or self-healing switch electrodes offer some advantage oversolid metal electrodes in that electrode pitting and consequentelectrode deterioration are avoided. Heretofore, mercury has by far beenthe preferred liquid metal electrode substance for switch applications.While mercury through the range of ambient temperatures, the vaporpressure of mercury is much too high for use with high voltage systems.The presence of mercury vapor increases arcing due to gas ionizationbetween the electrodes. De-

switches, earlier workers in high power capacity switch technology havenot dis-closed practical self-healing electrode switches for highvoltage, large current capacity service.

One technique extensively employed to increase switch hold-off voltageand to suppress switch arcing is to encase the contact electrodes andthe region the-rebetween in an evacuated envelope. The contactelectrodes are moved into and out of contact by mechanical linkageswhich interconnect through the vacuum seal. A remarkably reduced arcingbetween contact electrodes may be achieved by reducing the presence ofionizable gas in the envelope is normally in the liquid state r3,377,576 Patented Apr. 9, 1968 to a pressure below 10 mm. of Hg. As aresult, gas ionization breakdown is greatly reduced, however, the fieldemission arcing effects remain unchanged or may even increase in anevacuated environment. Thus, solid electrode pitting is not controlledby use of vacuum alone.

Although operation of a high power switch in vacuum achieves a markedreduction in electrode arcing, the mechanical motion of the electrodesin vacuum introduces heretofore unsolved mechanical problems. Mechanicallinkages interconnected through vacuum seals are notoriouslytroublesome. Previously available mechanical linkages operated throughvacuum seals exhibit only short useful life expectancy.

Still another problem exists in actuating mechanical linkages to closeor to open vacuum encased power switches. The mechanical movement of onemetallic member against another in high vacuum under conditions of largecurrent flow through the moving parts has been a continuous source ofswitch malfunction. Lubrication of the moving parts in a vacuum isrestricted to very low vapor pressure substances. The configuration ofthe switch linkages often requires that the lubricant be conductive forthe switch to function properly. Heretofore, none of the earlier highvoltage large capacity vacuum switches has satisfactorily avoided, withpractical inexpensive mechanical means, the disadvantage of drynon-lubricated sliding junctions between current conducting switchlinkage elements.

There remains, then, an unfulfilled need for a large current capacity.high hold-off voltage switch which simultaneously embodies theadvantages of self-healing electrodes, high vacuum environmentsuppression of ionized gas arcing and fully lubricated mechanical motionof the switch electrodes.

Accordingly, one object of the present invention is to provide animproved high hold-off voltage, large current capacity switch.

Another object of this invention is to provide a high voltage vacuumswitch with self-healing contact electrodes:

Still another object of this invention is large power capacity, highvoltage vacuum switch having lubricated sliding mechanical contacts.

Another object of this invention is to provide in a high voltage vacuumswitch system an improved means of communicating switch motion through avacuum seal.

Another object of this invention is to provide a high voltage, largepower capacity switch with a superior arc suppression capability.

Still another object of this invention is to provide a longlife, highlyreliable high voltage, high power capacity switch.

These and other objects and advantages of our invention will be evidentfrom the following illustrations, specification and claims.

FIGURE 1 is a partly cut away view of a first preferred embodiment ofour invention.

FIGURE 2 is a cross section view of a fragment of the embodiment of ourinvention shown in FIGURE 1.

FIGURE 3 is a partially cut away view of a second preferred embodimentof our invention.

FIGURE 4 is a partially cut away view of a variation of .the embodimentof our invention illustrated in FIG- URE 3.

FIGURE 5 is a URE 4 but showing two sliding electrodes.

Referring now to the figures, a first preferred emb0di ment of ourinvention illustrated in FIGURES l and 2 is adapted for interruption ofhigh voltage, large current capacity power mains. Stationary switchelectrodes 10 and 12 are mounted coaxially in a spaced relationshipwithin a gas tight electrically insulating envelope 14. The

to provide a partially cut away view similar to FIG-- envelope iscomprised of metal end caps 16 and 18, each having a central aperture 20and 22 through which the stationary electrodes and 12, respectively, aremounted. The apertures and 22 are each provided with a flange 24 and 26that provides a surface suitable for metallurgically bonding to therespective stationary electrodes 10 and 12.

A high dielectric material cylindrical tube 30 is sealed to the edges 32and 34, respectively, of the end caps 16 and 18 and provides the sidewalls of the gas tight envelope.

The electrical power main bus bars 36 and 38 are connected,respectively, by conventional mechanical means to the portion of thestationary electrodes 11) and 12 which extend outside the gas tightenvelope 14. In the embodiment shown in FIGURE 1, the stationaryelectrodes 10 and 12 are inserted through close fitting apertures in thebus bars 36 and 3-8.

Movable electrodes 40 and 42, comprised of high relative magneticpermeability metallic cylinders, are each respectively mounted coaxiallyover the stationary electrodes 10 and 12. The movable electrodes 40 and42 are sized to readily slide coaxially along the stationary electrodeswith only a small clearance. In addition, the movable electrodes aresufiiciently long so that when extended coaxially beyond the respectivestationary electrodes 10 and 12, the two movable electrodes 40 and 42will readily make contact with one another in the space between thestationary electrodes without slipping off the supporting stationaryelectrodes; and when the movable electrodes are retracted from the endsof the stationary electrodes, a substantial space exists between the twoends thereof. The relative position of the two movable electrodes 40 and42 in the closed switch position is illustrated in FIGURE 2, and theopen switch position is illustrated in FIGURE 1.

The movable electrodes 40 and 42 are normally held in the open switchpositions, such as illustrated in FIG- URE 1, by means of two tensionsprings 44 and 46 respectively mounted coaxially over the stationaryelectrodes 10 and 12. The spring 44 is connected at a first end to themoving electrode 40 and at the second end rigidly connects by means of acollar 48 to the end cap' 16. A groove is provided in the movableelectrode 12 and in the collar 48 into which one coil at each end ofspring 44 rests for purposes of securing the spring at either end.Similarly, the moving electrode 42 is normally held in a retracted oropen switch position by means of the spring 46 one end of which is fixedby a groove, shown in the illustrations, to the end cap 18 by means ofrigidly mounted collar 50.

FIGURE 2 illustrates the detailed structure of the movable electrodes 40and 42. The fixed electrodes 10 and 12 are made of copper and have atthe forward tips a flame spray tungsten coating 10a and 12a. The movableelectrodes 40 and 42 are made of ferromagnetic material such as 420'stainless steel, a high magnetic permeable stainless steel. The movableelectrodes are similarly provided with flame spray tungsten coatings 40aand 42a on those surfaces through which current may be required to pass.The tungsten coating furnishes a high wear resistant surface to theunderlying copper electrode. The electrical resistivity of tungsten ismuch greater than that of copper, however the flame spray coating may beas shallow as a few thousandths of an inch. Hence, in the arrangementshown in the illustrations, a relatively large area of the tungstencoating is available through which the current passes and onlynegligible resistances losses are encountered.

A solenoid 54 is mounted exterior of the envelope 14 about a highmagnetic flux capacity core 56. The core 56 is in turn mounted betweentwo magnetic flux conductors 58 and 60 which in turn are connectedrespectively to the stationary electrodes 10 and 12. When a directcurrent is passed through the solenoid coil 54, a magnetic flux isinduced in the core 56 and transmitted through the magnetic fluxconductors 58 and 60 to the stationary electrodes 10 and 12 and finallyto the movable electrodes 40 and 42. The polarity of the magnetic fieldtransmitted to the first movable electrode 40, by the above-describedarrangement, will always be opposite to that transmitted to the secondmovable electrode 42; thus, a strong magnetic field of opposite polaritymay be induced between the two movable electrodes. Amagnetic field ofsuitable strength will draw the movable electrodes together, overcomingthe spring tension which holds the two electrodes in normally separatedposition.

With the application of a large current to the solenoid 54, the movableelectrodes 40 and 42 may be brought together with considerable force. Toprevent damage to the stationary electrode mountings, cushion springs 62and 64 are mounted at the ends of the stationary electrodes 10 and 12 toabsorb the impact momentum incurred by the magnetic field force closingand the spring loaded opening of the movable electrodes.

Large current at high voltage must pass through the stationaryelectrodes 10 and 12 and the moving electrodes 40 and 42. By placing afilm 66 and 68 of liquid gallium metal over the stationary electrodes, alow viscosity, low vapor pressure, high conductivity contact surface ismaintained between the sliding metal electrodes. The gallium metal filmprovides a low friction lubricant having high conductivity and readilyhealed electrode action. Moreover, the vapor pressure of liquid galliummetal is lower than that of solid silver and compares favorably withcopper alloys. Accordingly, vacuum of 10- mm. Hg or better may bereadily maintained within the envelope 14. The physical properties ofgallium metal will not be described in detail here, but the reader isreferred to the numerous places in the literature which set forth thefull physical data.

By the above-described switch, large current at high voltages may bereadily and economically interrupted without appreciable arcing betweenelectrodes or without excessive wear between sliding electrode members.

Gallium melts at 30 C. Only a small quantity of heat need be applied tothe switch before start-up to assure that the gallium metal is meltedand is lubricating the moving electrode parts. After being in highvoltage, large current service for a short period of time, ample heat isnormally generated within the electrodes to assure liquification of themetallic gallium lubricant and electrode surfaces.

Referring now to FIGURE 3, a variation of our invention shown in FIGURES1 and 2 is illustrated. The actuation of the movable electrodes in theembodiment shown in FIGURE 3 is controlled by solenoidal magnetic fieldsapplied directly to the movable electrodes, as will be readilyunderstood from the following description.

Power main bus bars and 82 terminate on fixed electrodes 84 and 86. Thefixed electrodes are elongated and mounted in spaced relationship but incoaxial alignment. A vacuum tight envelope 88 is hermetically sealedabout an intermediate point on fixed electrodes 84 and 86 so that aportion of each electrode extends into the interior of the envelope 88,which is made of high dielectric, nonmagnetic material. The electrodesmay be made of any low resistance metal, copper being preferred. Movableelectrodes 90 and 92, comprised of hollow cylindrical tubes, are fittedover the fixed electrodes respectively and adapted to readily slidetherealong. The movable electrodes are made of a ferromagnetic,electrically conductive metal such as magnetic stainless steel. Coilsprings SP4 and 96 are mounted over the respective fixed electrodes andsecured at each first end of the respective springs to a movableelectrode and at the respective second ends of the springs to therespective fixed electrodes. The coil springs 94 and 96 will normallyhold the movable electrodes 90 and 92 in retracted positions; that is,out of contact with one another.

Solenoids 98 and 100 are positioned exterior of the envelope 88 andtransversally about the portion of the fixed electrodes 84 and 86 whichextend into the interior of the envelope 88.

A film of liquid gallium metal is coated onto the fixed electrodes 84and 86 and onto the surfaces of the movable electrodes 90 and 92. Thegallium metal readily wets other metal surfaces and liquefies at 30 C. Asmall quantity of heat from an external source or from a heating element102, inserted permanently within the envelope, may be required toliquefy the gallium metal electrode surfaces and lubricant prior to useof the switch.

The solenoids 98 and 100 are powered by direct current, the polarity ofwhich determines the polarity of the resultant magnetic field. If thepolarity of the solenoid magnetic fields from coils 98 and 100 areopposite in sign, the two movable electrodes 90 and 92 will have inducedtherein magnetic fields of opposite polarity and exert a strongattraction upon one another. Such attraction between the movableelectrodes will abruptly close the switch. The coil springs 94 and 96separate the movable electrodes, thereby opening the switch upon theremoval of the solenoidal magnetic fields.

FIGURE 4 illustrates still another embodiment of our invention. Twononmagnetic, fixed electrodes 110 and 112 are mounted to extend into ahigh dielectric nonmagnetic material envelope 114. The fixed electrodesare mounted in coaxial alignment but in a spaced arrangement so that aspace is provided between the fixed electrodes within the envelope. Theenvelope is preferably vacuum tight and capable of maintaining vacuum ofmm. Hg or greater.

A solenoid 116 is mounted on the exterior of the envelope 114 and whenenergized is positioned to induce a magnetic field within the envelopehaving a fixed polarity.

The fixed electrode 112 is provided with a cylindrical hollow recess 118into which a movable electrode 120 may be inserted. A coil tensionspring 122 secured at the end of the recess 118 is inserted into therecess 118 beneath the movable electrode 120. The second end of thespring 122 is secured to the end of the movable electrode as shown inthe illustration.

The movable electrode 120 is hollow and made of a nonmagnetic conductingmetal such as copper. Sealed mechanically within the electrode 120 is apermanent elongated maget 124 positioned so that the permanent magnetfield is coaxial with the movable electrode 120. The permanent magnetmay be made of any hard ferromagnetic material.

A film of gallium metal applied to the movable electrode 120 wets theexterior of the electrode 120 and the surfaces of the recess 118. Thegallium film when liquefied provides a low friction lubricant suitablefor relatively high vacuum environment between the moving electrode 120and the surfaces of the fixed electrode 112. Moreover, the liquidgallium film provides a highly conductive self-healing interelectrodecontact surface between the moving electrode parts.

When the solenoid 116 is energized, the resulting magnetic field reactson the permanent magnet 124 causing the movable electrode 120 to slidepartially out of the recess 118 and make contact with the first fixedelectrode 110. This action closes the switch. When the sole noid 116 isde-energized, the movable electrode 120 is abruptly retracted by thecoil spring 122. This latter action opens the switch. A variation ofFIGURE 4 is set forth in FIGURE 5. As shown, FIGURE 5 illustrates asecond sliding electrode controlled by a second solenoid, the parts ofwhich are designated by the same numerals as the sliding electrode andsolenoid in FIGURE 4 but with a sutfix a. The structure and function ofeach of the two sliding electrodes in FIGURE 5 are similar to thestructure and function of the single sliding electrode assembly shown inFIGURE 4.

As a variant of this embodiment, the fixed electrode can be replacedwith a second sliding electrode assembly including a solenoid, suchassembly being similar to the single assembly illustrated. The tips ofthe two sliding electrodes will preferably be formed int-o complementaryshapes for proper physical contact. In the resulting switch, the twomovable electrodes will readily make contact with one another in thespace between the stationary electrodes Without slipping out of therecesses therein; and when the movable electrodes are retracted from theends of the stationary electrodes, a substantial space exists betweenthe two ends thereof.

Numerous variations and alternative combinations of the detailedconstruction of embodiments of our invention fall within the scope ofour invention. The abovedescribed and illustrated preferred embodimentsof our invention are intended as merely illustrative of our invention,the scope of which is set forth in the following claims.

We claim:

1. An electrical switch comprised of an evacuated high dielectricmaterial envelope; a first and a second elongated fixed electrodemounted in coaxially aligned spaced relationship within the envelope; afirst and a second hollow cylindrical sliding electrode mounted,respectively, coaxially about the first and second fixed electrodes, thesliding electrodes being made of relatively high magnetically permeablematerial; a lubricating selfhealing film of gallium metal wetting theelectrode surfaces; and solenoids mounted exterior to the envelope andtransversally of the fixed electrodes; whereby, electrical contactbetween the fixed electrodes may be made and broken by alternatelyaxially moving the sliding electrodes into and out of contact in thespace between the fixed electrodes.

2. An electrical switch comprised of an evacuated high dielectricmaterial envelope; a first and a second elongated fixed electrodemounted in coaxially aligned spaced relationship within the envelope; afirst and a second sliding electrode mounted, respectively, to andcoaxially with the first and second fixed electrodes, the slidingelectrodes having means for mounting a permanent magnetic polarity; alubricating self-healing film of gallium metal wetting the electrodesurfaces; heating means for liquefying the gallium metal mounted withinthe envelope; and magnetic means mounted exterior of the envelope forpositioning the sliding electrodes along the fixed electrodes; whereby,electrical contact between the fixed electrodes may be made and brokenby moving the sliding electrodes into and out of contact in the spacebetween the fixed electrodes.

3. An electrical switch comprised of an evacuated high dielectricmaterial envelope; a first and a second elongated fixed electrodemounted in coaxially aligned spaced relationship within the envelope,the first such fixed electrode being characterized by having a hollowcentral recess having an aperture opening into the space between thefixed electrodes; a sliding cylindrical electrode slidably mountedwithin the recess, such sliding electrode being of relatively highmagnetically permeable material; a lubricating self-healing film ofgallium metal wetting the electrode surfaces; and magnetic means forpositioning the sliding electrode along the first fixed electrode andrelative to the second fixed electrode, being mounted to the envelope;whereby electrical contact between the fixed electrodes may be made andbroken by alternately moving the sliding electrode into and out ofcontact with the second fixed electrode.

4. An electrical switch comprised of an evacuated high dielectricmaterial envelope; a first and a second elongated fixed electrodemounted in coaxially aligned spaced relationship within the envelope; afirst and a second sliding electrode having a hollow tubularconfiguration mounted respectively coaxially on the first and secondfixed electrodes, the sliding electrodes being made of relatively highmagnetically permeable material; a lubricating self-healing film ofgallium metal wetting the electrode surfaces; and magnetic means forpositioning the sliding electrodes along the fixed electrodes; whereby,electrical contact between the fixed electrodes may be made and brokenbyalternately axially moving the sliding electrodes into and out ofcontact in the space between the fixed electrodes.

5. An electrical switch comprised of an evacuated high dielectricmaterial envelope; a first and a second elongated fixed electrode beingmounted in coaxially aligned spaced relationship within the envelope'andhaving each a hollow central recess with an aperture opening into thespace between the fixed electrodes; a first and a second slidingelectrode having cylindrical rod configuration, the first and secondsliding electrodes being mounted respectively coaxially within the firstand second fixed electrodes, the sliding electrodes being made ofrelatively high magnetically permeable material; a lubricatingselfhealing film of gallium metal wetting the electrode surfaces; andmagnetic means for positioning the sliding electrodes along the fixedelectrodes; whereby, electrical contact between the fixed electrodes maybe made and broken by alternately axially moving the sliding electrodesinto and out of contact in the space between the fixed electrodes.

6. An electrical switch comprised of an evacuated high dielectricmaterial envelope; a first fixed electrode, and a second fixedelectrode, the second fixed electrode having hollow cylindrical tubularconfiguration, the first and second fixed electrodes being mounted incoaxially aligned spaced relationship within the envelope; a'slidingelectrode mounted coaxially within the'second fixed electrode, thesliding electrode being made of relatively high magnetically permeablematerial; a lubricating self-healing film of gallium metal wetting theelectrode surfaces; and magnetic means for positioning the slidingelectrode along the second fixed electrode; whereby electrical contactmay be made and broken by alternately axially moving the slidingelectrode into and out of contact with the first fixed electrode in thespace between the fixed electrodes.

References Cited UNITED STATES PATENTS 1,948,687 2/1934 Swinne 200-166 X2,732,464 1/1956 Ohl 200166 2,749,402 6/1956 Tancerd 2.0087 3,150,9019/1964 Esten et al. 308241 OTHER REFERENCES German Printed Appl., Wesselet al., 1,116,816, Nov. 9, 1961.

BERNARD A. GILHEANY, Primary Examiner.

B. DOBECK, J. J. BAKER, R. N. ENVALL, JR.,

Assistant Examiners.

3. AN ELECTRICAL SWITCH COMPRISED OF AN EVACUATED HIGH DIELECTRICMATERIAL ENVELOPE; A FIRST AND A SECOND ELONGATED FIXED ELECTRODEMOUNTED IN COAXIALLY ALIGNED SPACED RELATIONSHIP WITHIN THE ENVELOPE,THE FIRST SUCH FIXED ELECTRODE BEING CHARACTERIZED BY HAVING A HOLLOWCENTRAL RECESS HAVING AN APERTURE OPENING INTO THE SPACE BETWEEN THEFIXED ELECTRODES; A SLIDING CYLINDRICAL ELECTRODE SLIDABLY MOUNTEDWITHIN THE RECESS, SUCH SLIDING ELECTRODE BEING OF RELATIVELY HIGHMAGNETICALLY PERMEABLE MATERIAL; A LUBRICATING SELF-HEALING FILM OFGALLIUM METAL WETTING THE ELECTRODE SURFACES; AND MAGNETIC MEANS FORPOSITIONING THE SLIDING ELECTRODE ALONG THE FIRST FIXED ELECTRODE ANDRELATIVE TO THE SECOND FIXED ELECTRODE, BEING MOUNTED TO THE ENVELOPE;WHEREBY ELECTRICAL CONTACT BETWEEN THE FIXED ELECTRODES MAY BE MADE ANDBROKEN BY ALTERNATELY MOVING THE SLIDING ELECTRODE INTO AND OUT OFCONTACT WITH THE SECOND FIXED ELECTRODE.